This section provides background information related to the present disclosure which is not necessarily prior art.
High levels of charge dilution by excess air along with recirculated exhaust gas is considered as one of the technologies to further internal combustion engines' thermal efficiency beyond engine downsizing.
For a lean combustion engine, the three-way catalyst (TWC) that is commonly used on stoichiometric engines is unable to control NOx effectively to meet the increasingly stringent exhaust emissions regulations. In order to manage NOx, an SCR (selective catalytic reduction) catalyst system is also required. One class of the SCR technology that is particularly attractive for lean combustion engine application is a passive system. The passive SCR catalyst can be applied as a standalone system or be complimented by a downsized urea-based system with the advantages of reduced packaging requirement and lower system cost.
The passive SCR system encompasses a TWC upstream and an SCR catalyst downstream in the exhaust system. The engine can be momentarily operated at a slightly rich equivalence ratio to use the TWC to generate ammonia which can be stored in the downstream SCR catalyst. The engine is then operated at the intended lean equivalence ratios for the desired thermal efficiency benefits and depends on the SCR catalyst to control NOx. When the stored ammonia in the SCR catalyst is depleted, the engine is switched to the ammonia generation mode again. By cycling between the lean and rich modes, both the efficiency and emissions control goals can be met.
To achieve the levels of dilution to match that desired for optimum combustion process, high levels of boost are required over a broad engine operating range including light-load conditions. Numerous advanced boosting solutions have been engineered with some being implemented for production. Essentially, all the advanced boosting solutions entail compound boosting of multiple turbochargers of different sizes or a turbocharger in combination with a supercharger. However, the use of multiple turbochargers or a turbocharger in combination with a supercharger can add undesirable cost and packaging complexities.